Method of stabilizing permanent magnetic material powders

ABSTRACT

Method of stabilizing cobalt-rare earth material having a superior coercive force by contacting the particles with zinc or arsenic in an inert atmosphere at a temperature about the melting point of metal.

United States Patent [72] Inventors [54] METHOD OF-STABILIZING PERMANENT MAGNETIC MATERIAL POWDERS 10 Claims, No Drawings [52] 11.8. CI 148/101, 148/3151 [51] Int. Cl H011 l/06 [50] Field of Search.... 148/101,

105,122 MM,31.51/117/100Mt234;75/212 [56] References Cited UNITED STATES PATENTS 1,726,340 8/1929 Buttles 29/192 2,306,198 12/1942 Verweij et al 117/234 Primary Examiner- Benjamin R. Padgett Assistant Examiner-Brooks H. Hunt Attorneys-Paul A. Frank, Charles T. Watts, Jane M.

Binkowski, Frank L. Neuhauser, Oscar B. Waddell and Melvin M. Goldenberg ABSTRACTzMethod of stabilizing cobalt-rare earth material having a superior coercive force by contacting the particles with zinc or arsenic in an inert atmosphere at a temperature about the melting point of metal.

METHOD OF STABILIZING PERMANENT MAGNETIC MATERIAL POWDERS The present invention relates generally to the art of making permanent magnets and is more particularly concerned with new magnetic material powders having unique characteristics and a novel method for producing these powders, and with magnets in which these powders are incorporated in substantially nonmagnetic matrices.

Permanent magnet properties of bulk magnetic materials having large magnetocrystalline anisotropies can be enhanced by reducing them to powders. Such powders can be incorporated in bonding media to provide composite permanent magnets having properties substantially superior to those of the bulk source materials. These advantages are offset to a substantial degree in some materials when the particle size reduction is accomplished by a method, such as grinding, which deforms or destroys the crystal structure of the material to decrease its coercive force. However, the detrimental effects of mechanical reduction of cobalt-rare earth materials can be eliminated and their coercive force increased to a surprising extent by chemical treatment. As disclosed in copending U.S. Pat. applications, Ser. No. 701,840, filed Jan. 31, 1968, now U.S. Pat. No. 3,558,372 and U.S. Ser. No. 730,577, filed May 20, 1968, now U.S. Pat. No. 3,558,371 the coercive force of cobalt-rare earth materials is significantly increased by treating them with an acid or an acid solution.

When good properties are attained in the cobalt-rare earth powders, whether by grinding alone or by chemical means, they tend not to be stable. As the powders are exposed to air at slightly elevated temperatures, their coercive force decreases irreversibly. This is especially true of chemically prepared or treated powders, in which the decay in coercive force even at room temperature takes place at an appreciable rate. This decay in coercive force takes place on heating even in a purified inert atmosphere. Thus, a comparaitively low value of coercive force can substantially diminish the advantages to be gained by converting the bulk body to a powder and fabricating a composite finished article from the powder.

The process of the present invention makes the magnetic properties of particles of cobalt-rare earth materials stable. in some cases it also substantially improves their magnetic properties. The process comprises contacting the cobalt-rare earth particles with a metal vapor or molten metal. Specifically, the cobalt-rare earth particles are heated in a substantially inert atmosphere to the temperature at which the metal vaporizes or becomes molten. The metal may deposit on the surfaces of the cobalt-rare earth particles or diffuse into them. The metals which are suitable are zinc and arsenic.

The process can be carried out by a number of conventional techniques. Zinc, in particulate form, for example, can be admixed with the cobalt-rare earth particles, and the mixture heated in an inert atmosphere to a temperature sufficient to vaporize the zinc so that it may contact the surfaces of the cobalt-rare earth particles. By another technique, a film of zinc may be deposited on the inside wall of a tube, such as a quartz tube, and the cobalt-rare earth particles placed into the tube and heated therein in an inert atmosphere to a temperature which vaporizes the zinc so that it can contact the surfaces of the cobalt-rare earth particles.

The amount of zinc or arsenic used to treat the cobalt-rare earth particles in the process of the present invention may vary widely. The particular amount is determinable empirically. It depends largely on .the degree to which the resulting zinc or arsenic treated particles are stable in air at room temperature, or at higher temperatures, and also upon the effect that such amount of metal may have on the coercive force of the resulting particles. Specifically, if the process conditions cause the zinc or arsenic only to deposit on or react with the surface of the coblt-rare earth particles, then the preferred maximum amount of the metal is an amount sufiicient to form a continuous film on the surface of the particle to envelop it. Amounts of metal in excess of such film-forming amount may be used but provide no significant advantage. However, if the process conditions cause the zinc or arsenic to diffuse into the particle, such diffused zinc or arsenic should be limited to an amount which does not significantly decrease the coercive force of the resulting cobalt-rare earth particle. The minimum amount of metal with which the particles need be treated is any amount which is sufficient to stabilize the material in air, i.e. that amount which imparts to the material the property of substantially retaining its high coercive force after prolonged exposure to air.

The particle size of the cobalt-rare earth compound used in the instant process may vary. it can be in as finely divided a form as desired. For most applications, it will range from about 325 mesh or less to about l00+l70 mesh, (U.S. standard screen sizes). Larger sized particles can be used, but as the particle size is increased, the maximum coercive force obtainable is lower because the coercive force generally varies inversely with particle size.

Representative of the cobalt-rare earth compounds useful in the process of the present invention are Co Sm, Co,Y and Co,M (cerium-rich misch metal).

The temperature at which the process is carried out must be sufficient to convert the zinc into molten or vapor form, or the arsenic into vapor form, so that in such form it can come into contact with the surfaces of the cobalt-rare earth particles. The hot zinc or arsenic may deposit on the surfaces of the particles, or ti may diffuse through such surfaces. The contact period may vary widely and is determinable empirically by the stability imparted to the treated cobalt-rare earth particles under a particular set of conditions.

The present invention is carried out in a substantially inert atmosphere, Le. a nonoxidative atmosphere. Such an atmosphere may be a vacuum or it may be, for example, hydrogen or an inert gas such as helium.

While no limitation on the claims is intended, it is believed that the results obtained by the present process can be explained on the basis that adsorbed substances initially present on the surfaces of the cobalt-rare earth particles might react with them at elevated temperatures and be responsible for the damage to coercive force. It is theorized that the action of these adsorbed impurities is inhibited by a reactive metal, such as zinc or arsenic, acting as a getter.

All parts and percentages used herein are by weight unless otherwise noted.

The invention is further illustrated by the following examples.

In all of the following examples, the coercive force of the cobalt-rare earth powder was measured at room temperature in the same manner. Specifically, a specimen of the cobaltrare earth powder was prepared for magnetic measurement by introducing it into a body of molten paraffin wax in a small glass tube and cooling the wax in an aligning magnetic field of 20,000 oersteds until the paraffin solidified. The intrinsic coercive force of each such prepared sample was then measured after magnetization in a field of 30,000 oersteds.

EXAMPLE 1 An ingot of cobalt-samarium (Co Sm) was ground with mortar and pestle. The resulting powder was screened and the fraction passing through a-325 mesh screen was selected for test. The intrinsic coercive force of this material was 7,900 oersteds.

A portion of the powder was placed in a porcelain boat in a quartz tube. A film of zinc had previously been deposited on the inside wall of the tube and surrounded the sample. The sample in the tube was flushed for 24 hours at room temperature in hydrogen that had been purified by being passed over heated copper and through a liquid nitrogen trap. The tube was then placed in a furnace preheated to 450 C. and maintained at this temperature for 10 minutes with the hydrogen still flowing slowly. It was then removed and cooled to room temperature. The intrinsic coercive force of this treated powder was 1 1,800 oersteds.

A sample of the treated powder and a sample of the powder as ground, i.e. not treated with zinc, were placed in an oven in air at 1 15 C., and after various periods of time in the oven, specimens of both samples were removed and their coercive force was measured. At the end of 1% hours in the oven, the intrinsic coercive force of the as-ground material was 5,400 oersteds, and after 144 hours of such heat-aging, it had decreased to 3,150 oersteds. The intrinsic coercive force of the zinc-treated cobalt powder, on the other hand, was entirely unchanged after 144 hours in the oven.

EXAMPLE 2 The procedure of this example was the same as that disclosed in example 1 except that the flushing time was reduced to 1 hour. The intrinsic coercive force of the resulting zinctreated material was 11,200 oersteds. After 126 hours in air at 1 15 C., this value remained exactly the same.

EXAMPLE 3 the procedure of this example was the same as that disclosed in example 2, except that the gas was helium. The intrinsic coercive force of the resulting zinc-treated. material was 12,200 oersteds. After 15 hours in air at 115 C., this value was unchanged.

EXAMPLE 4 The procedure of this example was the same as that disclosed in example 3 except that arsenic was used instead of zinc. The resulting intrinsic coercive force was 7,250 oersteds. This remained unchanged after heating the powder in air at 1 15 C. for 16 hours. Under these particular conditions the intrinsic coercive force of the material was not actually increased, but the desired stability was imparted to it.

EXAMPLE 5 the procedure used in this example was the same as that disclosed in example 1 except that the flushing time was reduced to minutes, the furnace temperature was 470 C. and the Co Sm was not in a boat but was near the supply of zinc in the tube, i.e. the particles were in contact with the film of zinc previously deposited on the inside wall of the tube. The intrinsic coercive force of the zinc treated powder was 14,500

oersteds. The high coercive force indicates that some molten zinc probably contacted the particles.

EXAMPLE 6 A batch of C0,, Sm, which was ground to pass through a 400 mesh screen, was immersed in a room temperature solution consisting of three parts HN0 one part H 80 one part H PO.,, and five parts CH COOl-l. At the end of 30 seconds, the powder was removed from the acid solution, rinsed with water and with acetone, and dried in air. This acid-treated powder had an intrinsic coercive force of 15,100 oersteds. The powder was contacted with zinc in the same manner as disclosed in example 1 except that the flushing time was 15 minutes, and the temperature of the furnace was 470 C. The zinc-treated powder, which had an intrinsic coercive force of 15,900 oersteds, was placed in an oven in air at 115 C. for 15 hours. At the end of this heat-aging period, its coercive force was measured and found to be unchanged.

By contrast another batch of Co,Sm powder, which was ground to the same size, was treated with the same acid solution in the same manner as above. It had an intrinsic coercive force of 17,000 oersteds. This acid-treated powder was heataged in air at 1 15 C. in the same manner as above. After onehalf hour at 1 15 C its intrinsic coercive force had gone down to 10,000 oersteds; after 2 hours of such heat-aging, it was 7.950 oersteds, and after 4 hours 6,400 oersteds.

EXAMPLE 7 Co -,Sm powder, which was ground to pass through a 325 mesh screen, was used. it had an intrinsic coercive force 6,700 oersteds. A sample of the Co -,Sm powder was weighed and mixed with a weighed amount of zinc powder. The zinc powder comprised 0.44 percent by weight of the mixture. This powder mixture was placed in a glass vial, and in a vacuum of about 10 mm, Hg it was heated to 470 C. for 10 minutes, then cooled to room temperature and weighted again. The difference between the final weight and the sum of the original weights was negligibly small.

The zinc-treated powder which, after treatment, had a coercive force of 7900 oersteds was placed in an oven in air at 1 15 C., and after 15 hours of such heat-aging, showed no change in coercive force indicating complete stabilization.

EXAMPLE 8 A batch of cobalt-yttrium (Co Y), which was ground to pass through a 400 mesh screen, was treated with the same acid solution in the same manner as disclosed in example 6. it had a coercive force of 6,900 oersteds.

If this acid-treated powder is treated with zinc, in the same manner as disclosed in example 6, it is expected that the zinctreated powder would have an intrinsic coercive force of about 6,000 oersteds which would remain substantially unchanged after being heat-aged in an oven in air at 1 15 C. for 15 hours.

EXAMPLE 9 A batch of Co M, where M is cerium-rich misch metal, which was ground to pass through a 400 mesh screen, was treated with the same acid solution in the same manner as disclosed in example 6. it had an intrinsic coercive force of 5,070 oersteds.

If this acid-treated powder is treated with zinc in the same manner as disclosed in example 6, it is expected that the zinctreated powder would have an intrinsic coercive force of about 5,000 oersteds which would remain substantially unchanged after being heat-aged in an oven in air at 1 15 C. for 15 hours.

EXAMPLE 10 The process of example 1 was repeated except that cadmium was used instead of zinc. After treatment for 10 minutes at 450 C., with cadmium, the coercive force had decreased from 7,900 to 1,530.

EXAMPLE 1 l The process of example 1 was repeated except that magnesium was used instead of zinc. After treatment for 10 minutes at 450 C., with magnesium, the coercive force had decreased from 7,900 to 2,750 oersteds.

The magnetic material produced by the process of the present invention can be incorporated in various bonding media to provide composite permanent magnets. Representative of such bonding media are polymers, especially epoxy resins, elastomers including natural rubber, and nonmagnetic metals.

Although the present invention has been described in connection with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and the appended claims.

What we claim as new and desire to secure by Letters Patent of the US. is:

l. A method of producing a particulate cobalt-rare earth material having a superior coercive force which is substantially stable in the presence of air which comprises providing a cobalt-rare earth material comprised substantially of a Co R intermetallic compound, where R is a rare earth metal, in the 5. The method of claim 1 in which the inert atmosphere is a form of particles and contacting said particles with a metal vacuum. selected from the group consisting of zinc and arsenic in an 6. The method of claim I in which the inert atmosphere is amount sufiicient to stabilize the material without significantly hydrogen. reducing the coercive force in an atmosphere in which said 5 The method of c131"! 1 which the almmphere cobalt-rare earth material is substantially inert at a temperahehhmlure above h melting point f said memh 8. The method of claim I wherein the zinc IS in vapor form.

2 h h d f l i 1 i which the compound is y 9. The method of claim 1 wherein the zinc is in molten form. 3 The method f l i 1 in which the compound is COM 10. The method of claim I wherein the arsenic is in vapor 4. The method of claim a in which the compound is Co M formwherein M is cerium-rich misch metal. 

2. The method of claim 1 in which the compound is Co5Y.
 3. The method of claim 1 in which the compound is Co5Sm.
 4. The method of claim 1 in which the compound is Co5M wherein M is cerium-rich misch metal.
 5. The method of claim 1 in which the inert atmosphere is a vacuum.
 6. The method of claim 1 in which the inert atmosphere is hydrogen.
 7. The method of claim 1 in which the inert atmosphere is helium.
 8. The method of claim 1 wherein the zinc is in vapor form.
 9. The method of claim 1 wherein the zinc is in molten form.
 10. The method of claim 1 wherein the arsenic is in vapor form. 